Re-investigations of the fossil fern Xiajiajienia mirabila (Dicksoniaceae) based on new material from the Lower Cretaceous of western Liaoning, China

Abstract

As one of the essential Mesozoic floristic elements, the family Dicksoniaceae experienced two flourishing periods, one in the Middle Jurassic and the other in the Early Cretaceous. Several fossil genera that yield cup-shaped or bivalvate indusia in sori have been considered members of this family. Among them, the genus Xiajiajienia Sun, Zheng et Mei 2001 presents unique characteristics with unlobed drepaniform sterile pinnules, marginal sori and bivalvate indusia. However, its systematical investigation was poorly documented due to the limit preservation of fossil specimens, which would compromise its taxonomic significance. Here, we report new fertile pinnae of Xiajiajienia mirabila from the Jianshangou Bed, Yixian Formation in western Liaoning, China. Based on the distinguishable characters of the newly-found specimen, a minor emendation of species diagnosis is proposed. A phylogenetic analysis was performed with a molecular scaffold, suggesting Xiajiajienia had a closer relationship with both modern and fossil Dicksonia. Previous research on the Yixian Formation's paleoenvironment (Jehol biota) suggests that X. mirabila lived in a cool, temperate mid-latitude climate with seasonal variations, especially in a humid area near water. The genus was suggested to be endemic to the Jianshangou flora of the Jehol biota, along with other genera in Dicksoniaceae, which witnessed the second flourishing period of the family around the world back to the Early Cretaceous.